This invention relates to servo systems for accessing and following defined servo tracks, and, more particularly, to servo systems having compound actuators comprising a coarse actuator and a fine actuator mounted on the coarse actuator.
In data recording on data storage media having high track density, such as magnetic tape or optical disk, a compound actuator, which comprises a coarse actuator and a fine actuator mounted on the coarse actuator, provides both a large working dynamic range and high bandwidth. Thus, the data head can be translated between tracks over a full width of the magnetic tape or between the inner and outer tracks of an optical disk, using the coarse actuator, and can track follow the movement of a track, using the fine actuator.
In the typical compound actuator, the fine actuator function follows the track guiding disturbances, as determined by a servo sensor which senses servo tracks, to position the data head in the center of the desired data track or tracks. It has relatively small mass and wide bandwidth and is thus able to follow high frequency disturbances. However, it has a very limited range of travel in order to provide the high bandwidth. The fine actuator servo system typically has a compensator function which is designed to enable maximum bandwidth with adequate stability margins. Examples are Chow et al., U.S. Pat. No. 5,090,002, and Fennema et al., U.S. Pat. No. 5,060,210. The coarse actuator carries the fine actuator from track to track and also aids in centering the fine actuator, typically by having a relative position sensor to sense if the fine actuator is substantially in the center of its range with respect to the coarse actuator on which it is mounted. Thus, if the fine actuator drifts substantially off-center during track following, the relative position sensor detects this movement and causes the servo system to correct the coarse actuator in the direction that the fine actuator has moved off-center.
As the heads and actuators become increasingly miniaturized, a difficulty with a relative position sensor is that the space in which to mount the relative position sensor and the electronic circuits associated with it becomes too small. Additionally, the cost of such a precise, high resolution sensor is detrimental.
An object of the present invention is to allow tracking of the fine actuator by the coarse actuator while eliminating the relative position sensor.
A servo system and method are disclosed for positioning a head laterally with respect to defined servo tracks, employing fine and coarse actuators. A servo control integrates the position error between the head and a desired position in accordance with a predetermined function, and operates the coarse actuator to translate the fine actuator and the head in accordance with the integrated position error. In one embodiment, a servo sensor senses lateral position of the head with respect to the defined servo tracks. A position error signal loop is coupled to the servo sensor for determining position error between the head and the desired position related to the defined servo tracks, for operating the fine actuator to translate the head in a manner to reduce the determined position error. The position error signal loop has a compensator function that includes an integrator and other transfer function elements such as a lead-lag function. The coarse servo control, for operating the coarse actuator, comprises a connection coupled to the position error signal loop compensator function for providing control to the coarse actuator from the position error integration signal representing the integration function of the position error signal loop; and a driver coupled to the connection, operating the coarse actuator to translate the fine actuator and the head in accordance with the integration signal.
For a fuller understanding of the present invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings.